The importance of achieving highly accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such accurate alignment difficult both to achieve and to maintain. For example, one issue of concern in the construction of laser transmitters is that of efficiently coupling the optical output from a laser diode into an optical fiber. To obtain efficient coupling, the fiber end is desirably precisely aligned with the emitting area of the laser. Precise alignment of addition optical components between the laser diode and the optical fiber, such as lens, modulators, or optical isolators, may also be desired. When such alignment is achieved, the fiber is then fixed in place, desirably by a method that enables the desired alignment to be sustained throughout the device lifetime.
Typically, fiber-coupled diode lasers are packaged in metal butterfly packages, which may be gold plated, and the fiber is held in alignment with the laser using one of the epoxy, laser weld, or solder attachment techniques with or without a ferrule. Epoxy attachment is low cost but may have too much thermal expansion for high precision attachments. Furthermore, it may not be reliable over a long period of time due to outgassing and alignment shifts arising from aging and temperature cycling. Laser weld techniques are reliable but use costly ferrulization of the fiber and specially designed mounts or clips to allow weld attachment of the ferrulized fiber. The mounts/clips are expensive, large, and may creep over time. Solder attachment techniques, on the other hand, are reliable and low cost, and have become prevalent in the art. Existing solder attachment techniques however, tend to use an integrated heating mechanism and/or a specially configured platform to isolate the heat used for solder reflow. These thermal management means may be expensive and/or undesirably large.
The mounting point at which the fiber is soldered desirably has specific material properties in order to work effectively. The material of the mounting point desirably has a low thermal conductivity (e.g. less than 50 W/m-K) and a sufficiently low thermal expansion coefficient to substantially maintain fiber alignment when the package is heated during operation, or due to environmental effects. The exact thermal expansion property desired may depend on a number of parameters including: the material to which the laser is mounted; the respective thickness of the fiber mount and laser submount; and/or the temperature profiles expected during operation. The fiber mount material also may be able to be soldered or be able to be plated with a solderable material. During the soldering process, the fiber mount may experience significant stress resulting from differential expansion due to temperature gradients and materials differences. Therefore, the fiber mount desirably has a high tensile strength (e.g. greater than 25 kpsi) to avoid fracturing.
It is difficult, however, to maintain alignment between the optical component and the fiber when the fiber is soldered due to turbulent flows and capillary forces exhibited by the molten solder. Other methods of post-solder adjustments may include the undesirable addition of costly and complicated tooling (e.g., grippers) and high precision linear or rotary stages. The pending U.S. patent application Ser. No. 10/858,894 to Singh et al., entitled NON-MECHANICAL ADJUSTMENT OF AN OPTICAL FIBER TO AN OPTICAL OUTPUT, addresses some of these problems by a method of aligning an optical fiber to an adjacent optical source by adjusting the fiber at least vertically toward a desired position. Therefore, the invention to Singh et al. teaches alignment by moving the optical component in only one direction by localized destressing of the solder used to hold the optical fiber.
To achieve the desired alignment accuracies for the optical coupling of many optical components it may be necessary for the optical components to be precisely aligned along two, or three, axes. For these optical component attachments, a method of performing multi-directional alignment is needed.